DESCRIPTION (Verbatim from Investigator's Abstract): Experiments proposed are aimed at determining structural similarities of drug binding sites between channel types for mibefradil (the only highly specific agent for T-type Ca channels known at the present time) and phenylalkylamines, the drug class that competes with mibefradil, and for which some structural information is available for L-type calcium channels. Three channels will be used, the T-type Ca channel alpha lH, as a high affinity target for mibefradil and a somewhat lower affinity target of verapamil, the voltage-gated Na channel, a very low affinity target for mibefradil but a moderate affinity target for verapamil, and HERG, a high affinity target for both mibefradil and verapamil. In each area, experiments proposed combine site-directed mutagenesis, electrophysiology, and molecular modeling. In addition, experiments are proposed that will establish the channel specific or common effects on kinetics that control the action and efficacy of these agents as therapeutic drugs.